There are currently in the market a number of zirconium oxide depositing conversion coating products intended to replace zinc phosphate products in automotive assembly lines. These known products are used to coat ferrous metal surfaces as well as aluminum and zinc containing surfaces.
An issue for current zirconium oxide conversion coating processes is that they do not provide as good corrosion protection as conventional zinc phosphate processes under some circumstances and on some substrates.
One particular problem has been that current processes for zirconium oxide conversion coating of zinc-containing substrates, in particular HDG and EG, do not provide coated metal articles that meet certain automotive testing requirements. Hot Dip Galvanized (HDG) and Electro Galvanized (EG) steel substrates treated with a commercially available zirconium oxide pretreatment baths exhibited weaker performance in two separate automotive industry corrosion tests when compared to the conventional Bonderite® zinc phosphate treatments currently used in the automotive industry. Thus there is a need for a process of coating these substrates with a zirconium oxide conversion coating that provides for improved corrosion performance of zirconium oxide conversion coatings on HDG and EG substrates.
The instant invention solves one or more to the above problems in zirconium oxide coating processes by including at least one pre-treating step, where the zinc-containing surfaces are contacted with a composition, for example a cleaner and/or a pre-rinse, containing cobalt and/or iron, in the zirconium oxide conversion coating processes.